Multi-ferroic structural health monitoring systems and methods

ABSTRACT

A method for testing a multiple layer structure is described. The method includes monitoring strains experienced by strain sensitive, magnetic particles disbursed within an adhesive between a first layer and a second layer, and analyzing any changes in the strains experienced by the particles to determine an amount of damage or degradation to the structure.

BACKGROUND

The field of the disclosure relates generally to structural healthmonitoring of composite structures, and more specifically, to methodsand systems for multi-ferroic structural health monitoring.

Composite materials are attractive for aerospace structural applicationsbecause of their high strength to weight ratios compared to metals.Particularly, composite materials are increasingly utilized in a widevariety of applications, including for aircraft structures. Compositeaircraft structures include mixtures of bonded and bolted laminates witha variety of metallic and composite substructures. Other examplesinclude composite sandwiched structures and other adhesive bonded panelsincluding assemblies and structures with contoured surfaces.

Further improvements in composite structure affordability can berealized through low-cost and light weight structural health monitoringsystems. Degradation of the composite laminate itself, due to impacts,lightning damage, thermal damage, or micro-cracking can be managed withcondition-based-maintenance, reducing costs for non-destructiveinspection (NDI), especially in limited access areas where disassemblyis required.

Non-destructive evaluation (NDE) methods currently used to inspectbonded structures include modified pulse-echo ultrasonic testing (UT),infrared thermography, shearography, eddy current testing, and varioushigh and low frequency ultrasonic methods.

Structural health monitoring (SHM) of composite and bonded structureshas focused primarily on networks of piezo-electric sensors sending andreceiving ultrasonic waves that interrogate the structure, or embeddedfiber optic sensors measuring strain, moisture, temperature, and thelike. Traditional nondestructive inspection methods rely uponquantifying defects within the structure to predict the flightworthinessof the structure. However, these inspection methods may be unable toascertain the cohesiveness of the bond at any location absent defects.Further, piezo-electric and fiber optic-based SHM methods havelimitations related to the embedding of the sensors, connectivity of thesensors, and costs for the supporting systems.

It remains desirable to inspect such structures to identify any defects,such as cracks, discontinuities, voids, or porosity, which couldadversely affect the performance of the structure.

BRIEF DESCRIPTION

In one aspect, a method for testing a multiple layer structure isprovided. The method includes monitoring strains experienced by strainsensitive, magnetic particles disbursed within an adhesive between afirst layer and a second layer, and analyzing any changes in the strainsexperienced by the particles to determine an amount of damage ordegradation to the structure.

In another aspect, a structural health monitoring system for multiplelayer structures is provided. The system includes a plurality ofstrain-sensitive magnetic particles dispersed within a bonding areabetween at least two individual layers, at least one multiferroic sensordispersed on a surface of the multiple layer structure, proximate to anarea of the structure to be monitored, the at least one multiferroicsensor capable of monitoring strains experienced by the strain-sensitivemagnetic particles, and a controller configured for periodicinterrogation of the at least one multiferroic sensor.

In still another aspect, a composite structure is provided. Thestructure includes a first layer, a second layer, an adhesive layerbetween the first layer and the second layer, and a plurality ofstrain-sensitive magnetic particles dispersed within the adhesive layer.

The features, functions, and advantages that have been discussed can beachieved independently in various embodiments or may be combined in yetother embodiments further details of which can be seen with reference tothe following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of an aircraft production and servicemethodology.

FIG. 2 is a block diagram of an aircraft.

FIG. 3 is a schematic illustration of a multi-ferroic structural healthmonitoring system for a structure.

FIG. 4 is a block diagram of a multiferroic sensor.

DETAILED DESCRIPTION

The described embodiments generally include three elements:magnetostrictive particles incorporated into a composite structure,multi-ferroic sensor array appliqués applied to the composite structure,and a controller with wireless communication, for interrogating thesensors within the array.

Referring more particularly to the drawings, embodiments of thedisclosure may be described in the context of aircraft manufacturing andservice method 100 as shown in FIG. 1 and an aircraft 200 as shown inFIG. 2. During pre-production, aircraft manufacturing and service method100 may include specification and design 102 of aircraft 200 andmaterial procurement 104.

During production, component and subassembly manufacturing 106 andsystem integration 108 of aircraft 200 takes place. Thereafter, aircraft200 may go through certification and delivery 110 in order to be placedin service 112. While in service by a customer, aircraft 200 isscheduled for routine maintenance and service 114 (which may alsoinclude modification, reconfiguration, refurbishment, and so on).

Each of the processes of aircraft manufacturing and service method 100may be performed or carried out by a system integrator, a third party,and/or an operator (e.g., a customer). For the purposes of thisdescription, a system integrator may include, without limitation, anynumber of aircraft manufacturers and major-system subcontractors; athird party may include, for example, without limitation, any number ofvenders, subcontractors, and suppliers; and an operator may be anairline, leasing company, military entity, service organization, and soon.

As shown in FIG. 2, aircraft 200 produced by aircraft manufacturing andservice method 100 may include airframe 202 with a plurality of systems204 and interior 206. Examples of systems 204 include one or more ofpropulsion system 208, electrical system 210, hydraulic system 212, andenvironmental system 214. Any number of other systems may be included inthis example. Although an aerospace example is shown, the principles ofthe disclosure may be applied to other industries, such as theautomotive industry.

Apparatus and methods embodied herein may be employed during any one ormore of the stages of aircraft manufacturing and service method 100. Forexample, without limitation, components or subassemblies correspondingto component and subassembly manufacturing 106 may be fabricated ormanufactured in a manner similar to components or subassemblies producedwhile aircraft 200 is in service.

Also, one or more apparatus embodiments, method embodiments, or acombination thereof may be utilized during component and subassemblymanufacturing 106 and system integration 108, for example, withoutlimitation, by substantially expediting assembly of or reducing the costof aircraft 200. Similarly, one or more of apparatus embodiments, methodembodiments, or a combination thereof may be utilized while aircraft 200is in service, for example, without limitation, to maintenance andservice 114 may be used during system integration 108 and/or maintenanceand service 114 to determine whether parts may be connected and/or matedto each other.

The description of the different advantageous embodiments has beenpresented for purposes of illustration and description, and is notintended to be exhaustive or limited to the embodiments in the formdisclosed. Many modifications and variations will be apparent to thoseof ordinary skill in the art. Further, different advantageousembodiments may provide different advantages as compared to otheradvantageous embodiments. The embodiment or embodiments selected arechosen and described in order to best explain the principles of theembodiments, the practical application, and to enable others of ordinaryskill in the art to understand the disclosure for various embodimentswith various modifications as are suited to the particular usecontemplated.

Turning now to FIG. 3, a schematic illustration of a multi-ferroicstructural health monitoring system 300 for a structure 302 is provided.Structure 302 includes a first composite layer 310, a second compositelayer 312, and an adhesive layer 314 therebetween that is typicallyreferred to as a bondline 314. It should be noted that first compositelayer 310 and second composite layer 312 may be fully cured compositeskin, laminate or sandwich layers with the bondline 314 being anadhesive for holding the layers together or plies of composite materialstacked together to form a laminate with bondline 314 being a layer ofresin in and around the fibers or fiber tows utilized in a compositefabrication process.

System 300 includes strain-sensitive magnetic particles 320 dispersedwithin the bondline 314, distributed as evenly as possible, multiferroicsensors 322 for monitoring strains experienced by strain-sensitivemagnetic particles 320 and a central controller 324 for periodicallyinterrogating the multiferroic sensors 322 and analyzing changes in theresponses to interrogation that may indicate damage or degradation inthe structure 302.

The magnetic signature of magnetostrictive particles 320 placed withinbondline 314 or elsewhere in the structure 302 is sensitive to thestrain induced by growing cracks, corrosion or delaminations, and evenweakened bonds. It has been demonstrated that loading of the structure302 increases the strain at these defects/degradation locations, therebyincreasing the magnetic signature and defect sensitivity of theparticles 320 near these damage locations. Similarly, the static anddynamic loading experienced by structures similar to structure 302during flight operations can produce or increase the magnetic responseand defect sensitivity of the magnetostrictive particles 320 within abondline such as bondline 314. The described embodiments utilize thisapproach, and with the application of multiferroic sensors 322 to astructure define the structural health monitoring system 300 for bondedand composite structure 302.

In one embodiment, the multiferroic sensors 322 are located at a surface330 of the structure 302 close to a critical area to be monitored. As anexample only, a multiferroic sensor may be dispersed on an outer surfaceof the first composite layer 310 or the second composite layer 312. Inembodiments, and as shown in FIG. 4, these sensors 322 incorporate RFIDdevices 332 as well as sensor elements 334. A power supply 340 such asenergy harvesting elements and/or batteries, a data processingcapability 342, a memory 344 are combined with RFID devices 332 andsensor elements 334 to provide a wireless installation with thecapability to store information indicative of the environment sensed bythe sensor element 334 and communicate such information to centralcontroller 324 through RFID interrogation.

Multiferroic sensor elements 334 are very sensitive magnetic sensorelements. As such, an array of sensors 322, each sensor 322 having amultiferroic sensor element 334, can provide very high sensitivity andresolution to pinpoint even very small defects that might occur withinbondline 314. In the embodiments described herein, the multiferroicsensor elements 334 are configured to either sense the magnetizationlevel of the magnetostrictive particles 320 or to sense the magneticpermeability of the particles 320.

As mentioned above, strain-sensitive magnetic particles 320 aredistributed throughout the bondline 314 as evenly as possible. In oneembodiment, strain-sensitive magnetic particles 320 are about 0.01 inchapart such that very small defects (as small as 0.01 inch) proximatebondline 314 can be sensed. Other configurations where the particles 320are farther apart are contemplated, depending on the application. In allsuch embodiments, sensors 322 are capable of sensing changes in thesignatures of strain-sensitive magnetic particles 320 over an area thathas a radius of about four inches, and are distributed accordingly, andto ensure overlapping coverage. Additionally an orientation of themagnetic particles 322 within the structure can be controlled byapplying an electric field during fabrication of the structure.

In addition, multiferroic sensor elements 334 are simpler to design andmanufacture and are more sensitive than alternative magnetic sensorssuch as coils, magneto-optical effect sensors and pick-up coils.

In the currently illustrated embodiments, and referring back to FIG. 3,multiferroic sensors 322 are located on an adjacent surface 350 of thestructure 302 containing the bondline 314. As such, multiferroic sensors322 are operable to monitor, measure, and in embodiments, store themagnetic signature caused by the local strains proximatemagnetostrictive particles 320, with very high resolution andsensitivity.

As easily understood, an array of these multiferroic sensors 322 areintegrated into a magnetic signature monitoring system, so more than onebond location can be monitored. In embodiments, computerized controller324 periodically interrogates each sensor 322, for example using RFIDtechnology, looking for symptoms of potential structural healthproblems. In one embodiment, sensors 322 are fabricated using MEMStechnology to reduce sensor size and integrate other capabilities suchas energy harvesting from ambient vibrations and energy storage.

Along with various types of damage or degradation, some types of weakbonds are detectable and can be monitored using the describedembodiments because the induced strain in the adhesive of bondline 314is sensitive to the modulus of adjacent materials such as low moduluscontaminants. Such a monitoring method is useful as an indicator oflocal failure when plastic strains in previously weak bonds are sensed.

Current structural health monitoring (SHM) approaches utilizepiezo-electric arrays or fiber optic sensor arrays for damage detection.However, embedded or attached piezo-electric sensor element arrays canindicate impact damage in composite structure, but are not typicallysensitive to bondline degradation. Such sensor arrays also require highvoltages in order to generate ultrasonic waves that interrogatestructure. Their presence, if embedded, may adversely affect thestructural performance. Fiber optics are gaining interest as SHMsensors, but, the attachment equipment interface using fiber optics iscomplicated and costly. Other magnetic sensors types that could be usedfor SHM are not as sensitive as multiferroic sensors. Existingmagnetometers which are able to achieve high levels of sensitivity areknown as superconducting quantum interference devices (SQUIDs). SQUIDsmust operate at cryogenic temperatures, which requires a large andcostly setup. As a result, many applications that would benefit fromhigh-sensitivity magnetometry, have not been possible outside of awell-equipped lab.

System 300, as described above, allows real time detection andmonitoring of weak bonds or smaller unbonds, with small, lightweight,distributed sensors sensing changes in the magnetic signaturesassociated magnetostrictive particles 320 with low power consumptionrequirements while taking advantage of present wireless interfacecapabilities.

The magnetostrictive particles 320 are located in the bondline 314between layers of composite material or between cured compositestructures or within the composite structure. Redundant arrays of smallmultiferroic sensors 322 are attached to the adjacent surfaces and havea wireless (such as RFID) communications capability. Wireless devicessuch as communications controller 324 are utilized for communicationwith individual sensors 322 as well as one or more data collection andstorage computers (not shown).

The magnetostrictive particles 320 are fabricated from amagnetostrictive metal, alloy or ferrite material. Materials that arehighly magnetostrictive in nature, such as Terfenol-D or nickel, have ahigher sensitivity to strain. The particles 320 may be of any shape buthigh aspect ratio shapes such as flakes or fibers have better mechanicalcoupling to the adjacent structural materials and are therefore moresensitive to strain in the structure. Other high aspect ratio forms suchas thin coatings of magnetostrictive material on nonmagnetic fibers(such as fiberglass or carbon) are also effective. The magnetostrictiveparticles 320 may be a wide range of sizes smaller than the thickness ofthe bondline and are preferably much smaller in order to minimize anydetrimental effects of the particle on the bond strength.

The redundant arrays of multiferroic sensors 322 may include the sensorelements, drivers, energy harvesting devices and a digital memory deviceas described above. The sensors 322 may include both magnetic andacoustic sensor elements. The magnetic sensors 322, in embodiments, aredisbursed in an array that is as large as necessary or practical tocover the bondline area to be monitored. The magnetic sensors 322include elements that are based on multiferroic technology. In a typicalapplication, the array of sensors 322 may include hundreds or thousandsof individual sensors that are typically less than 100 microns across.Each sensor 322 may be made of one or more layer pairs ofmagnetostrictive and piezoelectric materials to form a multiferroicdevice. An array may include additional sensors such as piezoelectricsensors to detect mechanical vibrations. The array may also includemagnetic drivers (not sensors) that produce a time varying magneticfield or mechanical vibrations in the bondline 314. The sensorelectronics correlate the imposed magnetic and or mechanical signal withthe sensed signals to interpret the strain in the bondline 314 fordefects or degradation. In one embodiment, the drivers and sensors arepowered by batteries which may be charged by energy harvestingpiezoelectric devices that harvest ambient mechanical vibrations.

The sensor arrays, electronics and wireless communications devices mayall be incorporated within thin film devices mounted on a flexibleappliqué that is attached to the surface using pressure sensitiveadhesives or other convenient approaches. The data collection andstorage computer associated with controller 324 periodically, or ondemand, interrogates some or all of the sensor arrays using wirelesscommunications and alone, or in conjunction with an associated computer,tracks history and sensed changes with the structure.

In other embodiments, the strain-sensitive magnetic particles 320 aredispersed throughout the structure, for example, aircraft skin panels,stringers, and the like. As described above, at least one multiferroicsensor 322 is dispersed on a surface of the structure, proximate to anarea of the structure to be monitored. The multiferroic sensor(s) 322are capable of monitoring strains experienced by the strain-sensitivemagnetic particles 320, The controller 324 is configured for periodicinterrogation of the multiferroic sensor(s) 322

A testing process, for example, may include: generation of magnetic andor mechanical vibrations by one or more drivers, reaction of themagnetostrictive particles 320 to the vibrations that is dependent onthe local strain, sensing of the reaction by the sensors 322, receivingdata from sensors 322 at controllers 324, transmission of the sensordata from controller 324 to a post processing device, and processing ofthe data from each sensor 322 in the array to form a high resolutionimage of the bondline 324. Storage and wireless transmission of theimage data to the computer along with correlation of the image withhistorical data is then utilized to monitor structural health of thecomposite structure.

The generated magnetic vibration may be at any frequency ranging fromnear DC to Gigahertz. Frequencies in the range of a few hundred Hertzare preferable because attenuation is low. The magnetostrictiveparticles 320 react to the magnetic vibration (by virtue of theirmagnetic permeability) by partially magnetizing and demagnetizing at thesame frequency. The permeability of the magnetostrictive particlesaffects the magnetic field at the sensor elements generating adetectable voltage in the sensor that is proportional to the magneticpermeability of the magnetostrictive particles. The detected signal alsodepends on the local strain because of the magnetostrictive propertiesof the particles 320.

The local strain depends on the presence or absence of structuraldefects including weak and unbonded joints. In this way the structuralhealth is monitored. The effect of defects on the local strain isaccentuated by application of a mechanical vibration either from ambientvibrations or from mechanical vibrations generated by mechanicaldrivers. Use of ambient vibrations has the virtue of simplicity but useof generated vibrations may lead to improved signal to noise ratios (andthus to improved sensitivity to defects) because signal processingtechniques can correlate the mechanical and magnetic signals to minimizemagnetic noise that may arise from nearby electronics or magneticmaterials. The generated mechanical vibration may be at low levels so asnot to cause damage or interfere with other subsystems. The mechanicaldrivers may be located with the sensor array or may be from a distantsource.

In other embodiments, the strain sensitive, magnetic particles 320 maybe dispersed throughout the multiple layer structure. The strainsexperienced by such particles may be compared with a baseline fordetermination of any manufacturing defects in the multiple layerstructure.

The signal from each sensor element may be used to form a strain imageof the bondline 314, or structure, using conventional image processingtechniques. The resolution may be on the order of the size of the sensorelements which may be less than 100 microns and as small as 10 microns.The sensor array may image the bondline 314 periodically or asinstructed by the data collection and storage computer(s). The image istransmitted to the computer, compared to historical data and reportsissued to maintenance personnel.

This written description uses examples to disclose various embodiments,which include the best mode, to enable any person skilled in the art topractice those embodiments, including making and using any devices orsystems and performing any incorporated methods. The patentable scope isdefined by the claims, and may include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral languages of the claims.

What is claimed is:
 1. A method for testing a multiple layer structure,said method comprising: monitoring strains experienced by strainsensitive, magnetic particles dispersed within an adhesive between afirst layer and a second layer; and analyzing any changes in the strainsexperienced by the particles to determine an amount of damage ordegradation to the structure.
 2. The method according to claim 1 whereinmonitoring strains comprises periodically interrogating multiferroicsensors dispersed on a surface of the structure.
 3. The method accordingto claim 2 wherein analyzing any changes in the strains analyzingchanges in the responses to interrogation.
 4. The method according toclaim 2 wherein periodically interrogating multiferroic sensorscomprises interrogating individual multiferroic sensors using RFID. 5.The method according to claim 2 further comprising dispersing theindividual multiferroic sensors to provide overlapping coverage for thestrain sensitive, magnetic particles.
 6. The method according to claim 1further comprising dispersing the strain sensitive, magnetic particlesat least 0.01 inch apart in the adhesive throughout an area of thestructure to be monitored.
 7. The method according to claim 1 whereinmonitoring strains comprises at least one of sensing a magnetizationlevel of the magnetic particles and sensing a magnetic permeability ofthe magnetic particles.
 8. The method according to claim 1 furthercomprising monitoring strains experienced by strain sensitive, magneticparticles dispersed throughout the multiple layer structure.
 9. Themethod according to claim 8 further comprising comparing the monitoredstrains with a baseline to inspect for manufacturing defects in themultiple layer structure.
 10. A structural health monitoring system formultiple layer structures comprising: a plurality of strain-sensitivemagnetic particles dispersed within a bonding area between at least twoindividual layers; at least one multiferroic sensor dispersed on asurface of the multiple layer structure, proximate to an area of thestructure to be monitored, said at least one multiferroic sensor capableof monitoring strains experienced by said strain-sensitive magneticparticles; and a controller configured for periodic interrogation of theat least one multiferroic sensor.
 11. The structural health monitoringsystem of claim 10 wherein said controller is programmed to analyzechanges in responses to interrogation from said at least onemultiferroic sensor that may indicate damage or degradation in thestructure.
 12. The structural health monitoring system of claim 10wherein said plurality of strain-sensitive magnetic particles aresubstantially evenly dispersed within the bonding area.
 13. Thestructural health monitoring system of claim 10 wherein saidmultiferroic sensors are dispersed on the surface of the multiple layerstructure to provide overlapping coverage of said plurality ofstrain-sensitive magnetic particles.
 14. The structural healthmonitoring system of claim 10 wherein said at least one multiferroicsensor is configured to sense at least one of a magnetization level ofsaid magnetic particles and a magnetic permeability of said magneticparticles.
 15. The structural health monitoring system of claim 10wherein said at least one multiferroic sensor comprises an RFIDcommunications capability for responding to said controller.
 16. Thestructural health monitoring system of claim 10 wherein each saidmultiferroic sensor comprises: a data processing capability; a memory;an RFID communications capability; and a multiferroic sensor element,each said multiferroic sensor capable of storing information indicativeof the environment sensed by said sensor element, and communicating suchinformation to said controller through RFID interrogation.
 17. Thestructural health monitoring system of claim 16 wherein each saidmultiferroic sensor comprises at least one of a battery and an energyharvesting device.
 18. The structural health monitoring system of claim10 wherein said plurality of strain-sensitive magnetic particlescomprise at least one of a magnetostrictive metal, a magnetostrictivealloy, and a magnetostrictive ferrite material.
 19. The structuralhealth monitoring system of claim 10 wherein said plurality ofstrain-sensitive magnetic particles comprise a high aspect ratio shapefor coupling to the structure and increased sensitivity to strain in thestructure.
 20. The structural health monitoring system of claim 10wherein said plurality of strain-sensitive magnetic particles comprisenonmagnetic fibers having a coating of magnetostrictive materialthereon.
 21. The structural health monitoring system of claim 10 furthercomprising a flexible appliqué for application to the surface of thestructure, said flexible appliqué comprising said at least onemultiferroic sensor fabricated as a thin film device attached thereto.22. A composite structure comprising: a first layer; a second layer; anadhesive layer between said first layer and said second layer; and aplurality of strain-sensitive magnetic particles dispersed within saidadhesive layer.
 23. The composite structure of claim 22 wherein saidfirst and said second layers comprise a cured composite material, saidadhesive operable for holding said first and said second layerstogether.
 24. The composite structure of claim 22 wherein said first andsaid second layers each comprise plies of composite material stackedtogether with other layers to form a laminate, said adhesive layerbetween each ply comprising a resin utilized in a composite fabricationprocess.
 25. The composite structure of claim 22 further comprising atleast one multiferroic sensor dispersed on an outer surface of at leastone of said first layer and said second layer.